1. Field of the Invention
The invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to an HDD read amplifier/write driver integrated circuit that compensates for write head asymmetry.
2. Description of the Related Art
HDDs typically include a main integrated circuit, which is typically a system-on-a-chip (SOC) that contains many of the electronics and firmware for the HDD. The SOC is connected by a transmission line on a flex cable to the HDD's read amplifier/write driver (preamp), which is typically implemented as an integrated circuit. The preamp is typically located on the actuator that moves the read/write heads to the selected data tracks on the disks. The data to be written by the write head is sent from the SOC to the preamp, where the write driver generates analog positive and negative write current pulses that are applied to the inductive coil in the write head to write data by selectively magnetizing the magnetic media of the recording layer on the disk. The data read back from the disk by the read head is transmitted from the preamp to the SOC.
The write head will typically have inherent magnetic asymmetry, meaning that even if the positive and negative write current pulses are perfectly symmetric the magnetic fields applied to the disk will not be properly shaped. This will cause an asymmetric waveform when the data is read back, resulting in soft errors in the readback data. The inherent magnetic asymmetry of the write head is caused by variations in the manufacturing process, which results in variations in the shape of the write head.
To compensate for the inherent magnetic asymmetry of the write head, a higher write current or a variable overshoot current and/or variable overshoot current duration are required to generate an asymmetric write current waveform. The write current waveform has several parameters that can be controlled in the HDD by settings in the preamp. These waveform parameters include write current amplitude, write current rise and fall times, write overshoot current amplitude and write overshoot current duration. The write driver circuit in the preamp has dynamic switches to turn on these parameters with precision. U.S. Pat. No. 8,687,311 B2 describes a preamp that can adjust various parameters of the write current to alter the shape of the write current waveform to compensate for write head magnetic asymmetry.
However, to create an asymmetric write current waveform, these write parameters need to have high-speed asymmetric circuit current sources for each write parameter for high-speed writing. Although implementation of high-speed asymmetric current sources in the preamp is possible, the amount of circuitry involved to dynamically switch asymmetric write parameters increases the amount of integrated circuit area and slows down the generation of the write driver waveform.
What is needed is a preamp that can compensate for inherent write head magnetic asymmetry without directly altering the write current and thus without the need for high-speed asymmetric circuit current sources for each write parameter.